Antibody avidity effects the functional activity of anti-Haemophilus b PS antibodies, and therefore, avidity measurements could provide a better correlation between pneumococcal serum antibody concentration and protective immunity. Antibody avidity, which is an expression of the functional antibody affinity, may affect the protective efficacy of antibodies. The objective of the present study was to evaluate the avidity of antibodies in mice immunized with type 9V PS-protein conjugates (monovalent and within a 7-valent vaccine) and the 9V PS vaccine. The avidity index was expressed by the percentage of antibody that remained bound to the antigen, after potassium thiocyanate treatment. The avidity indices (AIs)of IgG antibody were highest in mice immunized twice with 7-valent conjugate, or with combined immunization with both 7-valent conjugate and 23-valent PS vaccine (AI of 89.0 - 91.1), followed by monovalent type 9V PS-protein conjugate (AI 66.6 - 75.7). The mice that were immunized with 9V PS exhibited lowest avidity (AI 52.2). Similar results were observed in the AIs of IgM antibody. These results showed that immunization with a polyvalent conjugate or polyvalent conjugate followed by a Ps vaccine induced high avidity antibodies. The serum concentrations of 9V PS IgG and IgM Abs were significantly higher in mice immunized with 9V PS-Ply, 9V PS-Aly conjugate after two booster doses, 7-valent conjugate twice, 7-valent conjugate followed by 23-valent PS vaccine compared to 9V PS alone or non-immunized controls. No significant increase in antibody response was observed in mice immunized with 23-valent PS vaccine followed by 7-valent conjugate, compared to the mice given 9V PS alone. Thus, priming mice with polyvalent conjugate vaccine has induced high antibody response compared to the group primed with polyvalent PS vaccine.